Retroreflectors as in my U.S. Pat. No. 3,663,084 have the property of returning an incident beam of light to its source. Lateral transfer retroreflectors reflect an incoming ray laterally through an offset distance and then reversely, parallel to the incoming ray. Such devices include two flat reflecting surfaces at right angles to each other forming a roof reflector and a third at right angles to each of the first two, displaced from the roof reflector by the required offset distance. Characteristically, the incoming ray and the outgoing ray are parallel without concern as to the exact relationship of the device to the path of the incoming ray.
The three flat reflecting surfaces could be formed on a single piece of glass, but at great cost because of the difficulty of making the three optically flat surfaces at right angles to each other, especially where there is a large lateral transfer distance. The device could more readily be fabricated of three separate flat reflecting plates and a central spacing number. Two of the plates could form a roof reflector, and the third reflecting plate could be carried by the central member at a distance from the roof reflector. However, many exacting requirements must still be met as to the shapes of some of the parts and as to the relationships of certain edges and surfaces of the parts.